State-of-the-art implantable medical devices are often equipped to measure impedance (or related electrical parameters such as admittance) between various pairs of electrodes implanted within the patient. Examples include intracardiac impedance measurements made between pairs of electrodes mounted to leads implanted on or within the various chambers of the heart. Other examples include intrathoracic impedance measurements made between the housing of the device (or “can” electrode) and electrodes implanted on or within the heart. Traditionally, such impedance measurements were deemed to be representative of the electrical impedance along a vector between the electrodes. That is, impedance measurements were associated with a particular pair of electrodes or some combination of three or more electrodes. Herein, these measurements are generally referred to as “vector-based” impedance measurements because the measurements are associated with at least one pair of electrodes and the vectors therebetween. In terms of analyzing and interpreting the measured impedance data, the interpretation typically relied on a conceptual model wherein the measured impedance was deemed to be representative of the impedance of the field between the electrodes pairs, including far-field contributions to that impedance. This traditional model is referred to herein as the “far-field model” of impedance. Under the far-field model, impedance measured along a vector between a pair of electrodes A and B is deemed to be representative of the field between A and B.
As one example of the far-field model, intrathoracic impedance measurements made between the device housing and a cardiac electrode implanted within the heart are deemed to represent the impedance to electrical flow spanning a field extending through the lungs between the device and the cardiac electrode. This intrathoracic vector-based impedance measurement is then used, for example, to assess pulmonary fluid congestion to detect pulmonary edema (PE) or heart failure (HF). Although this traditional interpretation of the impedance measurements can be useful, the present inventors have recognized that an alternative interpretation of impedance measurements based on a “near-field model” can provide a more useful means for understanding, analyzing and interpreting impedance measurements.
Briefly, with the near-field model, impedance parameters (or related electrical parameters such as admittance or immittance) are measured by an implantable medical device along vectors extending through tissues of the patient between various pairs of electrodes. The device then converts the vector-based impedance measurements into near-field individual electrode-based impedance values. This is accomplished, in at least some examples, by converting the vector-based impedance measurements into a set of linear equations to be solved while ignoring far-field contributions to the impedance measurements. The device solves the linear equations to determine the near-field impedance values for the individual electrodes, which are representative of the impedance of tissues in the vicinity of the electrodes. The device then performs or controls a variety of device functions based on the near-field values, such as analyzing trends in near-field values to detect HF or PE.
The present invention is directed to providing various additional systems and methods that exploit near-field measurements to, for example, assess heart chamber disequilibrium, electrode-tissue interface issues, etc. Various techniques are also set forth for calibrating near-field-based techniques.